This powers my Modded Fostex T50RP Mk 3 with no problem so I'm sure it will be fine for the 6xx. I also just got my Schiit Stack today and so far it's great. But The Xduoo is also portable and very easy to use with a phone or whatever. The main downside to the Xduoo is the drivers needed on Windows and Linux. The Schiit Stack is plug and play.

I run Antergos (based on Arch Linux) on my Ryzen desktop build and laptop, both can detect the XD-05 in USB DAC mode, plug and play basically. I use the XD-05 with Audacious using the ALSA plugin, bit-perfect playback, although there is some delay when switching between songs of different bit depth.